a) Field of the Invention
The invention is directed to a device with a laser for image presentation in which the laser emits laser light of a defined coherence length L at a given wavelength .lambda. and in which there is arranged in the path of the laser light a first structure with which phase displacements can be carried out for individual photons of the laser light in accordance with a predetermined distribution.
b) Description of the Relevant Art
The best known and, at present, most commonly used devices for lasers for image presentation are laser printers in which the information to Le printed is written on a light-sensitive cylinder by means of a laser beam, this cylinder being supplied with toner at the locations illuminated by the laser light, and the toner is then transferred to the paper for printing.
Other devices such as that known, e.g., from DE 195 01 525 C1, use the laser for sequential illumination of picture points of a television picture on a screen. Due to the inertia of the eye, the individual light points are averaged so that an observer perceives the image information as a video picture.
In both types of devices, lasers are used in particular in order to achieve a high dot or point resolution which can be achieved substantially due to the extensive parallelism of the laser beams. A further advantage of the laser over other light sources is the high energy density which is advantageous primarily in video systems of the type mentioned above so that it is also possible to present an image with suitably high light density or luminance on a very large projection surface with screen diagonals greater than 1.50 m or even on cinema screens.
The advantages of the laser in this respect are based on the stimulated emission of photons which, however, also leads to a high coherence of the laser beam emitted by the laser. However, this characteristic of coherence which is positive in other respects is troublesome in the presentation of images because it can lead to interference structures which are manifested in the presented image as scintillating dots. These speckles, also called interference phenomena, corrupt the reproduced image and cannot be tolerated for optimum image presentation.
The general survey article "Speckle Reduction in Coherent Information Processing", Toshiaki lwai and Toshimitsu Asakura, Proceedings of the IEEE, Vol. 84, No. 5, May 1996, mentions various possibilities for reducing speckle. Of particular interest in this article is a graph which shows that the number of publications has increased steadily from 1970 to 1990, which is a clear indication that no satisfactory solution has yet been found for speckle reduction.
The general survey article contains theoretical calculations for speckle reduction. Also, various methods are mentioned in which the spatial or temporal coherence of laser beams is disturbed. In particular, the premise consists in that the speckles are blurred due to local or spatial changes in the laser beam so that the contrast of the speckles is reduced.
A local interference of coherence was also attempted in DE 195 015 25 C1, which was already mentioned, by means of a phase plate. This phase plate is located in the path of the laser and acts upon different partial beams of the laser beam with different phases in the order of magnitude of the wavelength. In particular, the individual areas on the phase plate for generating the various phase differences are stochastically distributed so that it should be assumed that the phases of the individual partial light beams are distributed in a manner similar to the light from conventional light sources.
It has been confirmed by experiments that an appreciable reduction in speckle is possible with a phase plate of this kind. However, it has been observed that the individual structures in the phase plate which lead to a phase displacement of suitable magnitude give rise to new diffraction phenomena. Light bundles of all diffraction orders must therefore by collimated through a lens, but the beam product of the laser light is slightly deteriorated by this diffraction. It has further been observed that the raster or grid of the phase plate was detectable in the projected image, which indicates that in spite of the phase plate the speckle contrast was possibly still high enough to be detected by the eye.
The disadvantage of the reduced beam product could be overcome, however, when a screen with scattering bodies in which different phase displacements are generated based on different path lengths by static scattering is used instead of a separate phase plate. But tests have shown that such screens with path length differences in the order of magnitude of several wavelengths for different photons of the laser beam do not lead to the desired successful elimination of speckle.
Thus, it could be assumed that the laser light in which speckles occur differs substantially in still other physical characteristics from the light of other light sources in which no speckles were previously observed. Another physical magnitude for characterizing a light source is coherence length. Standard light generally has substantially shorter coherence lengths than laser light.
It is reported in WO 96/08116 that a pulsed laser at a pulse time of 1 ps, that is, with a coherence length of 0.3 mm, exhibited a substantially reduced speckle contrast compared with illumination of the same screen by a He-Ne laser. However, it cannot be known, a priori, whether this observed effect is attributable to the reduced coherence length or to the special construction of the laser. In addition, although the coherence length is changed by the pulses, every pulse, in order that a suitable light density can be generated at all, contains a substantially higher photon density than in continuous operation, so that interference through a large number of photons should even be increased. The only effect that could facilitate speckle reduction is based on the greater spectral width .DELTA..lambda.. But, as can be worked out by the known equation .DELTA..lambda.=.lambda..sup.2 /L, where L is coherence length, and taking into account the fact that the width of the interference maximum is substantially proportional to the wavelength .lambda., this spectral broadening cannot explain the observed reduction during pulses according to the previous understanding of speckle formation.
In particular, the measurement data in WO 96/08116 still show a small speckle structure. If the interpretation that the speckle structures substantially depends on the selected coherence length is correct, it should be possible to generate a similar speckle pattern with other light sources as well, e.g., such as a gas discharge lamp with similar coherence length (1 ps corresponds to L.apprxeq.0.3 mm). There is no knowledge of this aspect.
These considerations show that the occurrence of speckle is only poorly understood in practice, so that every method for speckle reduction relies essentially only on empirical knowledge.
This is disadvantageous in terms of technology in that a method for speckle reduction taken from the literature cannot necessarily be applied to different, even similar, devices. Due to the lack of a general teaching for the occurrence of speckle from which appropriate reduction mechanisms could be derived, it is even conceivable that a method for speckle reduction which happens to be effective in a prototype will cause insurmountable difficulties in large-scale manufacture. Thus, it cannot be assumed with absolute certainty that a sufficiently high reproducibility will be achieved with any of the known methods.